Here are some really easy things you can do to obtain a better image next time you go out there. Before you look for your next gear upgrade, make sure you get these right first.

If you use a DSLR, don't shoot in JPEG. You lose precision, you lose fidelity, and the camera performs on-board processing (such as stretching and sharpening) that is detrimental to signal quality later on.

Use darks & flats. Just do it. Your images will be much easier to process and instantly look 10x better.

Don't use in-camera noise reduction that is based on software (e.g. most of them). Chances are the noise reduction you can perform in post-processing is much better in preserving detail than the cheap&nasty noise reduction in your camera.

If you use a DSLR, chances are the resolution of your CCD is way too high for your seeing conditions. Your images will be blurry when zoomed in. Use this blur to your advantage by binning (not scaling!) your images down to the level where the blur is not noticeable anymore. You'll lose resolution but again a great amount of noise reduction. Alternatively undo the blur using deconvolution.

Take some time to get your focus right. A no/low-cost Bahtinov mask can really help, but using your DSLR with liveview can also help nail your focus.

If you shoot many subs for long periods of time, check your focus again in between frames after a while. Chances are you'll find it's slightly off again.

Don't apply deconvolution after stretching or after any other processing. It makes no mathematical sense. This is also a reason why you will want to have a signal that is as pure as possible (e.g. no JPEG, no stretching, no noise-reduction, just stacking) before you do this important step.

Properly polar align your mount. Keep at it. You'll get better at it.

Dither between subframes. Move the object by just a tiny bit, spiraling out from the initial position of your first sub.

When using a DSLR, trust the color data you captured. Get to know the influences that are causing discoloration (ex. light-pollution, amp glow), model them, subtract them and be left with the real signal. Color correction (other than saturation) should not be necessary when using an off-the-shelf DSLR without filters in the imaging train.

Hope this helps anyone.

Cheers,

Ivo

03-15-2012, 08:13 AM

Phil Leigh

Great list!

#11... If using a Canon DSLR that has 160 ISO multiples (160, 320,480,640) use them as they have the lowest sensor pattern noise. Avoid the 125 multiples as they have the highest noise. The 100 multiples (the so-called "native values") are in between.
The 160 multiples do a 1/3rd stop digital pull on the sensor data, whereas the 125 do a similar push (gain boost).
This barely matters in normal photography unless you are looking at low light shadow detail... But in AP everything is low light shadow detail :->

03-15-2012, 08:53 AM

Phil Leigh

Quote:

Originally Posted by Phil Leigh

Great list!

#11... If using a Canon DSLR that has 160 ISO multiples (160, 320,480,640) use them as they have the lowest sensor pattern noise. Avoid the 125 multiples as they have the highest noise. The 100 multiples (the so-called "native values") are in between.
The 160 multiples do a 1/3rd stop digital pull on the sensor data, whereas the 125 do a similar push (gain boost).
This barely matters in normal photography unless you are looking at low light shadow detail... But in AP everything is low light shadow detail :->

To be clear, this in-built "exposure compensation" happens before the RAW data is written in the camera, so cannot be avoided or post-corrected except by choosing the right ISO. For the 7D (for example) ISO 640 is the sweet-spot in terms of self-noise captured in RAW files. Each camera is different...

03-15-2012, 08:59 AM

Phil Leigh

#12 - (low cost if you already have one!) - if you have access to an AC outlet, use an AC adapter for a DSLR rather than internal batteries if you can... the lithium batteries get very slightly warm in AP use and this in turn warms up the sensor a little... this makes more noise!

03-15-2012, 04:35 PM

ghswen

Very good tips guys!

Thanks for posting.

03-15-2012, 05:26 PM

Phil Leigh

#13 - Don't forget to fit the supplied rubber eyepiece blind to the DSLR viewfinder during long exposures. The blind is normally to be found fitted to the original strap that came with your camera (it can be removed from the strap, but don't lose it - I've not yet been able to source a replacement).
The eyepiece surround needs to be removed first before fitting the blind - if you lose those, replacements are easy to find.

#14 - don't bother leaving a pause between multiple subs. It wastes your valuable time and the sensor won't cool down unless you leave a very long pause and it's better to have your subs at a constant temperature....and to have your lights and flats and bias frames at the same temp too.

#15 - if your DSLR has a "silent shooting" mode (i.e. fully electronic shutter release) then see if you can use it. This will reduce shutter-induced vibration ... even if you are also using mirror lockup - assuming you can do that on your camera. This is particularly useful on short subs (< 30 secs) - on longer subs the vibrations will die away quite quickly and be lost in the overall exposure, but I still think it is good practice.

03-15-2012, 05:28 PM

Celestron

Yeah, there are some awesome tips here. This should be stickied!

I'll add one:

#16: Never use the lowest f ratio, as you'll get bloated stars and distortions. Keep it at f3.2 to f5.6 for optimal results. This may differ camera to camera, so the best way to find out is to take a night just to figure out where your sweet spot is.

#11... If using a Canon DSLR that has 160 ISO multiples (160, 320,480,640) use them as they have the lowest sensor pattern noise. Avoid the 125 multiples as they have the highest noise. The 100 multiples (the so-called "native values") are in between.
The 160 multiples do a 1/3rd stop digital pull on the sensor data, whereas the 125 do a similar push (gain boost).
This barely matters in normal photography unless you are looking at low light shadow detail... But in AP everything is low light shadow detail :->

do you have any references for this info? I'm rather curious about this and would like to read up on it. The main reason I run a custom firmware on my 400D is to get more "inbetween" ISO values, so I'm wondering if this will help me like you're suggesting.... I'm thinking I may try ISO 640 with mine instead of the 800 I've been using, maybe that will help remove more noise from my frames.
excited now, some more reading ahead of me!

EDIT - I just did a bit of a google search, and I'm getting it now, a bit tricky to grasp at first. I'll try this thursday if the forecast doesn't change from now til then.

03-28-2012, 09:17 AM

tedrampart

sorry to post to this again when no one has replied since my last post.. but I'm wondering about the 160 multiples and if those that only have 100 multiples (like ISO 100,200,400,800)... would it be the same basic idea to just do a +-EV to accomplish the same task? which could be done on camera or in processing?